Apparatus and methods for controlling a switch mode power converter using a duty cycle state machine

ABSTRACT

A power conversion system includes a switch mode power converter including a switch coupled to regulate an output voltage to a regulation value in response to a feedback signal. A controller is coupled to deliver a gate drive signal to the switch during an enabled control condition. The controller is further coupled to inhibit the gate drive signal to the switch during an inhibited control condition. A duty cycle state machine included in controller is coupled to adjust a duty cycle of the gate drive signal according to a plurality of discrete duty cycle states.

BACKGROUND INFORMATION Field of the Disclosure

The present invention relates to controlling a switch mode power converter using a duty cycle state machine and more particularly to controlling a discrete duty cycle of a boost converter coupled to a secondary of a flyback converter.

Background

Many electronic devices, such as cell phones, laptops, etc., are powered by direct current (dc) power derived from a power supply. Conventional wall outlets generally deliver a high voltage alternating current (ac) power that is converted to regulated dc power in order to be used as a power source for consumer electronic devices. In some applications a power conversion system may cascade one or more power converter stages including a power factor correction (PFC) stage. Switch mode power converters are commonly used due to their high efficiency, small size, and low weight to convert a high voltage ac power to a regulated dc power.

One topology of a switch mode power converter is a flyback converter, which uses a transformer to transfer switched energy from a primary winding to a secondary winding. Another topology is a boost converter, which converts an input voltage to a boosted (higher) output voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments for controlling a switch mode power converter using a duty cycle state machine are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.

FIG. 1A illustrates an example power conversion system including a switch mode power converter and a duty cycle state machine according to an embodiment of the present invention.

FIG. 1B illustrates another example power conversion system including a switch mode power converter and a duty cycle state machine according to another embodiment of the present invention.

FIG. 2 illustrates an example switch mode power converter and controller using a duty cycle state machine according to an embodiment of the present invention.

FIG. 3 illustrates an example circuit realization of a gate control module according to an embodiment of the present invention.

FIG. 4 illustrates corresponding switching signal waveforms of the example gate control module of FIG. 3.

FIG. 5 illustrates an example state diagram of a duty cycle state machine according to an embodiment of the present invention.

FIG. 6 illustrates a conceptual flow diagram for controlling an example switch mode power converter using a duty cycle state machine according to an embodiment.

Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the teachings herein. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments for controlling a switch mode power converter using a duty cycle state machine.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth in order to provide a thorough understanding for controlling a switch mode power converter using a duty cycle state machine. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the teachings herein. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present disclosure.

Reference throughout this specification to “one embodiment”, “an embodiment”, “one example” or “an example” means that a particular feature, structure or characteristic described in connection with the embodiment or example is included in at least one embodiment for controlling a switch mode power converter using a duty cycle state machine. Thus, appearances of the phrases “in one embodiment”, “in an embodiment”, “one example” or “an example” in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures or characteristics may be combined in any suitable combinations and/or subcombinations in one or more embodiments or examples. Particular features, structures or characteristics may be included in an integrated circuit, an electronic circuit, a combinational logic circuit, or other suitable components that provide the described functionality. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale.

In the context of the present application, when a transistor is in an “off-state” or “off”, the transistor blocks current and/or does not substantially conduct current. Conversely, when a transistor is in an “on-state” or “on”, the transistor is able to substantially conduct current. By way of example, in one embodiment, a high-voltage transistor comprises an N-channel metal-oxide-semiconductor (NMOS) field-effect transistor (FET) with the high-voltage being supported between the first terminal, a drain, and the second terminal, a source. In some embodiments, an integrated controller circuit may be used to drive a power switch when regulating energy provided to a load. Also, for purposes of this disclosure, “ground” or “ground potential” refers to a reference voltage or potential against which all other voltages or potentials of an electronic circuit or integrated circuit (IC) are defined or measured.

As introduced above, a power conversion system may cascade one or more power converter stages including a power factor correction (PFC) stage. A modern configuration based on this concept may include a PFC stage using a flyback topology (i.e., a flyback converter) coupled to another switch mode power converter operating asynchronously with the flyback converter. For instance, the switch mode power converter may be a boost converter electrically coupled to a secondary winding of the flyback converter. In this way the boost converter can advantageously operate as an additional power conversion stage for regulating an output voltage at a load.

Additionally, when used as part of a cascaded system (e.g., a system having a flyback converter and a boost converter), the boost converter may advantageously use a simpler and/or more relaxed control approach. For instance, a control approach using a constant frequency boost switch drive with cycle skipping may be used to regulate the output voltage without sacrificing system regulation specifications. Using a constant frequency cycle skipping approach may advantageously simplify loop compensation and/or component requirements thereby improving overall loop response and dynamic performance. However, cycle skipping may also lead to audible noise. Accordingly, there is a need for a constant frequency cycle skipping control approach which eliminates and/or substantially reduces audible noise.

An apparatus and methods for controlling a switch mode power converter using a duty cycle state machine are disclosed herein. The duty cycle state machine may provide one or more signals to vary a discrete value of the duty cycle. This in turn may advantageously reduce cycle skipping and associated audible noise. Additionally, the duty cycle state machine may select the discrete duty cycle based on one or more system operating conditions.

FIG. 1A illustrates an example power conversion system 100 a including a switch mode power converter 108 and a duty cycle state machine 150 according to an embodiment of the present invention. The power conversion system 100 a includes a first power converter 101, a system controller 106, the switch mode power converter 108, and a load 105. System input power (i.e., input current I_(IN) and input voltage V_(IN)) may be delivered between input terminals 98 and 99, relative to a primary ground GND, and converted into system output power (i.e., output current I_(O) and output voltage V_(O)). System output power may be delivered to the load 110 between output terminals 105 and 107 relative to a local ground SGND.

The system controller 106 is electrically coupled to the first power converter 101 and the switch mode power converter 108 to control and regulate the transfer of energy delivered to the load 110. As illustrated, the first power converter 101 can provide an intermediate current I_(CC) to the load (e.g., a constant current) and also provide secondary power (e.g., supply voltage V_(R)) to an input of the switch mode power converter 108.

Also as shown, the first power converter 101 may use a flyback topology (i.e., a flyback converter) including a clamp 102, a transformer 104, secondary side components 103, and a rectifier block 109. The transformer 104 includes a first primary winding 112, a second primary winding 113, a first secondary winding 114, and a second secondary winding 115. In the depicted example, the system controller 106 includes a primary switch 132, a primary control module 120, and a secondary control module 124.

The primary control module 120 may control switching of the primary switch 132 to provide switched power (i.e., switched current I_(SW) and switched drain voltage V_(D)) at the primary winding 112, which in turn may magnetically couple the switched power to the secondary windings 114-115 and to primary winding 113. Coupled power at the primary winding 113 may be rectified with rectifier block 109 to provide a rectified supply voltage V_(BP) to the primary control module 120. Coupled power at the secondary windings 114-115 may be transferred to the load 110 through the secondary components 103.

Additionally, the primary control module 120 may receive one or more additional signals from within and external to the system controller 106. For instance, the signal V_(P), external to the system controller 106, may be provided to the primary control module 120 for under-voltage lockout (UVLO) and/or over-voltage (OV) protection. Also, the primary control module 120 may receive a signal FL from the secondary control module 124. The signal FL may be a galvanically isolated signal (e.g., a magnetically coupled and/or optically coupled signal) conveying control information from the secondary control module 124. In this way, the secondary control module 124 may be galvanically isolated from the primary control module 120 such that signals at the secondary control module 124 may be referenced to local ground SGND and isolated from signals at the primary control module 120, which are referenced to primary ground GND.

As illustrated, the secondary control module 124 is coupled to the secondary components 103. The secondary components 103 include a divider 151, a diode D1, a diode D2, a capacitor C1, and an NMOS switch 152. Also as shown, the divider 151 is electrically coupled to the secondary winding 115, and may provide a secondary winding signal V_(FW), useful for communicating information relating to the operation of primary switch 132.

In transferring power from the secondary winding 114, the secondary control module 124 may couple gate drive signal V_(HS) through capacitor C1 to the gate of NMOS switch 152 in order to block and/or to pass current (i.e., current I_(Cc)). For instance, coupled power at secondary winding 114 may be rectified with diode D2 and passed to the load 110 when the gate drive signal V_(HS) controls NMOS switch 152 to conduct current. Alternatively, coupled power at secondary winding 114 may be blocked from the load 110 when the gate drive signal V_(HS) controls NMOS switch 152 to block current.

Coupled power at secondary winding 115 may be rectified with diode D1 and transferred in order to supply intermediate power (i.e., power including supply voltage V_(R)) to the secondary control module 124 and to an input of the switch mode power converter 108.

As illustrated, the secondary control module 124 is also coupled to the switch mode power converter 108. The switch mode power converter 108 includes an inductor L1, a capacitor C2, a feedback network 153, a diode D3, and a switch 154 configured in a boost topology (i.e., a boost converter).

In transferring and regulating power from the secondary winding 115, the secondary control module 124 may control the switch mode power converter 108 to convert intermediate power (i.e., power including supply voltage V_(R)) and to provide the converted power to the load 110. The secondary control module 124 may receive a feedback signal V_(FB) from the feedback network 153 and in response provide a gate drive signal V_(GB) (e.g., a switching signal) to the gate of switch 154 to transfer power to the load 110.

According to the teachings herein, the secondary control module 124 may include the duty cycle state machine 150. The duty cycle state machine 150 may be used to control the on time (and/or the duty cycle) of the gate drive signal V_(GB), thereby reducing and/or eliminating audible noise.

FIG. 1B illustrates another example power conversion system 100 b including a switch mode power converter 108 and a duty cycle state machine 150 according to another embodiment of the present invention. The power conversion system 100 b is similar to power conversion system 100 a, except it includes a current sensing network 111, and it illustrates additional features of the load 110 and the secondary control module 124. For instance, as illustrated, the load 110 may include light emitting diodes 143. Additionally, the secondary control module 124 may include a controller 125. The controller 125 in turn may comprise the duty cycle state machine 150 and may be configured to control the switch mode power converter 108.

As shown, the current sensing network 111 may provide a sense signal V_(Is) to the secondary control module 124. The sense signal V_(Is) can be proportional to a drain/source current I_(SW) of switch 154 and/or proportional to output current I_(O). In this way the secondary control module 124 may provide an overcurrent feature to the power conversion system 100 b. For instance, when the sense signal V_(Is) exceeds a current sense threshold, then secondary control module 124 can adjust or inhibit gate control to switch 154.

FIG. 2 illustrates the switch mode power converter 108 and controller 125 using a duty cycle state machine 150 in accordance with an embodiment of the invention. The controller 125 may include a gate control module 204 and a buffer 206. A function of the controller 125 can be to control the switch 154 so that the switch mode power converter 108 converts power input (i.e., power including supply voltage V_(R)) into output power (i.e., output current I_(O) and output voltage V_(O)).

As illustrated the gate control module 204 can include the duty cycle state machine 150 and can provide a signal DB_(ON) to the buffer 206. According to an embodiment, the signal DB_(ON) can be a digital signal which may be buffered by buffer 206 to provide a gate drive signal V_(GB) to the switch 154 for controlling operation of the switch mode power converter 108. For instance, as shown the switch mode power converter 108 may be configured as a boost converter. According to an embodiment, the gate drive signal V_(GB) may be provided with constant frequency and constant duty cycle determined by the duty cycle state machine 150.

Also as illustrated, the controller 125 can receive the sense signal V_(Is), the feedback signal V_(FB), and also receive power from supply voltage V_(R). The gate control module 204 may receive the sense signal V_(Is) and/or the feedback signal V_(FB), and in response provide the signal DB_(ON) with a constant frequency and duty cycle determined, in part, by the duty cycle state machine 150. The feedback signal V_(FB) can be a voltage derived from a voltage divider formed by resistors R1 and R2 of the feedback network 153. As described above, the sense signal V_(Is) can be a signal indicative of the drain/source current I_(SW) and/or the output current I_(O).

In controlling the switch mode power converter 108, the controller 125 may enable and/or inhibit the gate drive signal V_(GB) based, in part, on the feedback signal V_(FB). For instance, according to an embodiment, when the feedback signal V_(FB) exceeds a reference value, then the controller 125 may inhibit the gate drive signal V_(GB) so that the switch 154 operates in a high-impedance (i.e., blocking) state. Alternatively, when the feedback signal V_(FB) falls below the reference value, the controller 125 may enable the gate drive signal V_(GB) to be provided to the gate of the switch 154 with constant frequency. In this way the controller 125 may control the switch 154 to regulate output voltage V_(O) by skipping pulses during an “inhibited” condition and by allowing pulses during an “enabled” condition.

Pulse skipping, which is also known as on-off control, can advantageously provide a stable, yet fast loop response. As introduced above, pulse skipping may unfortunately introduce audible noise. The duty cycle state machine 150 may be used to vary the duty cycle of the signal DB_(ON) so that audible noise is reduced and/or eliminated.

Although the embodiment depicted in FIG. 2 shows a controller 125 and switch mode power converter 108 configured in a boost topology, other configurations for use in a power conversion system and/or in a standalone switch mode power supply system are possible. For instance, the controller 125 and switch mode power converter 108 may be configured in other topologies including buck and buck-boost. Additionally, the controller 125 may be configured to use greater or fewer signals. For instance, the controller 125 may receive power from a voltage source other than the supply voltage V_(R). The controller 125 may receive greater or fewer signals than the feedback signal V_(FB) and the sense signal V_(IS). Also, in some configurations, the controller 125 may also provide more than just one gate drive signal V_(GB) to control multiple switches.

FIG. 3 illustrates an example circuit realization of the gate control module 204 according to an embodiment of the present invention. The gate control module 204 includes a triangle wave generator 300, a variable duty cycle square wave generator 301, a feedback comparator circuit 303, an overcurrent comparator circuit 304, a clock delay circuit 305, a RESET-SET (RS) latch 310, the duty cycle state machine 150, and a multiplexor 311. The variable duty cycle square wave generator 301 includes a square wave generator 302, a comparator 321, an OR-gate 307, a D-type flip flop (DFF) 306, and an AND-gate 309.

The triangle wave generator 300 is electrically coupled in a feedback configuration with the square wave generator 302 to generate a triangle signal V_(C). The triangle wave generator 300 provides the triangle signal V_(C) to the square wave generator 302, which, in response, generates an oscillator signal DB_(OSC) and an inverted oscillator signal DB_(OSCB). The oscillator signal DB_(OSC) and the inverted oscillator signal DB_(OSCB) are provided back to the triangle wave generator 300 in a positive feedback loop. By virtue of positive feedback the triangle signal V_(C) may assume a triangular waveform.

As illustrated the triangle wave generator 300 includes a current source 317, an NMOS transistor MN2, a capacitor 315, a current source 313, and an NMOS transistor MN1. A first terminal of the capacitor 315 is electrically coupled to local ground SGND. The second terminal of the capacitor 315 is electrically coupled to the source of NMOS transistor MN2 and the drain of NMOS transistor MN1. Also, as shown the NMOS transistor MN2 is electrically connected between the current source 317 and the second terminal of capacitor 315 to source and/or to block a current I2 to the capacitor 315 in response to the oscillator signal DB_(OSC) at the gate of NMOS transistor MN2. Additionally, the NMOS transistor MN1 is electrically connected between the current source 313 and the second terminal of the capacitor 315 to sink and/or block a current I1 from the capacitor 315 in response to the inverted oscillator signal DB_(OSCB) at the gate of NMOS transistor MN1.

The alternating charging of the capacitor 315 by the current I2 and the discharging of the capacitor 315 by the current I1 can generate the triangle signal V_(C). The triangle signal V_(C) can assume a triangular waveform based on the magnitude of current I1 and the magnitude of current I2. For instance, in one non-limiting embodiment, currents I1 and I2 may be substantially time independent and the triangle signal V_(C) may assume a triangular waveform. Additionally, in some embodiments the magnitude of current I1 can be substantially equal to a magnitude of current I2.

Also, as illustrated, the square wave generator 302 includes comparators 322-323 and a latch 325. The latch 325 includes NOR-gates 326-327, which are electrically cross-coupled to provide the oscillator signal DB_(OSC) and the inverted oscillator signal DB_(OSCB) in response to a set signal S1 and a reset signal R1. Comparator 323 provides the set signal S1 to NOR-gate 327 based on the comparison of the triangle signal V_(C) with a reference signal AV_(B). When the triangle signal V_(C), at the inverting input of comparator 323, intersects (e.g., falls below) the reference signal AV_(B), then the set signal S1 transitions high. In response, the oscillator signal DB_(OSC) transitions high (i.e., a logic high) and the inverted oscillator signal DB_(OSCB) transitions low (i.e., a logic low). When the oscillator signal DB_(OSC) transitions high, NMOS transistor MN2 sources current I2 onto capacitor 315 causing the triangle signal V_(C) to increase as a function of time.

The triangle signal V_(C) increases until the triangle signal V_(C) intersects (e.g., exceeds) a reference signal AV_(T). Comparator 322 provides the reset signal R1 to NOR-gate 326 based on the comparison of the triangle signal V_(C) with the reference signal AV_(T). When the triangle signal V_(C), at the non-inverting input of comparator 323, exceeds the reference signal AV_(B), then the reset signal R1 transitions high. In response, the oscillator signal DB_(OSC) transitions low and the inverted oscillator signal DB_(OSCB) transitions high. When the inverted oscillator signal DB_(OSCB) transitions high, NMOS transistor MN1 sinks current I1 from capacitor 315 causing the triangle signal V_(C) to decrease as a function of time.

FIG. 4 illustrates corresponding switching signal waveforms of the gate control module of FIG. 3. Waveform 402 may correspond to the triangle signal V_(C) (e.g., a voltage signal) of height dV (e.g., a height dV of one volt), and waveform 404 may correspond to the oscillator signal DB_(OSC). For instance, times t1 and t5 delineate when the triangle signal V_(C) intersects the value of AV_(B) (i.e., the reference signal AV_(B)) and when the latch 325 is set by comparator 323. At times t1 and t5 the oscillator signal DB_(OSC) (i.e., waveform 404) transitions high and the triangle signal V_(C) (i.e., waveform 402) increases as a function of time. Similarly, times t4 and t8 delineate when the triangle signal V_(C) has intersected the value of AV_(T) (i.e., the reference signal AV_(T)), and when the latch 325 is reset by comparator 322. Additionally, the period and frequency may be determined from times t1 and t5. For instance, the period may be determined by the difference of time t5 and time t1. In some embodiments, the frequency of signal DB_(OSC) may be substantially equal to two hundred kilohertz (200 kHz).

As illustrated in FIG. 3, the variable duty cycle square wave generator 301 may be used to provide (i.e., to enable) the signal DB_(ON) based on the feedback signal V_(FB). The signal DB_(ON) may be derived from a logical AND of a signal Q1 and the oscillator signal DB_(OSC). As shown the DFF 306 receives a signal D1 at its “D” input and provides the signal Q1 based in part upon the value of D1. For instance, the signal Q1, and hence the signal DB_(ON), may be forced to logic low (i.e., inhibited) when the signal D1 is low. Alternatively the signal Q1 may be clocked by the oscillator signal DB_(OSC) (i.e., enabled) to have a variable duty cycle waveform (e.g., waveform 408) when the signal D1 is a high. The circuit features giving rise to inhibiting and enabling the signal DB_(ON), based on the feedback signal V_(FB), may be traced to feedback comparator circuit 303.

The feedback comparator circuit 303 can include a comparator 333 which provides the signal D1 based on a comparison the feedback signal V_(FB) to a reference V_(R2). As illustrated, when the feedback signal V_(FB), at the inverting input, exceeds the reference V_(R2), the comparator 333 may assert the signal D1 to a logic low. When the feedback signal V_(FB) falls below the reference V_(R2), the comparator 333 may assert the signal D1 to a logic high. With reference to FIG. 2, the feedback signal V_(FB) can be indicative of the output voltage V_(O). Thus, the control of signal DB_(ON) relative to the reference V_(R2) as shown in FIG. 3 allows the regulation of the output V_(O) by pulse skipping (i.e., cycle skipping). When the feedback signal V_(FB) exceeds the reference V_(R2) the signal DB_(ON) becomes inhibited (i.e., low) thereby inhibiting the gate drive signal V_(GB). When the feedback signal V_(FB) reduces below the reference V_(R2) the signal DB_(ON) becomes enabled thereby enabling the gate drive signal V_(GB).

As shown in FIG. 3 the variable duty cycle square wave generator 301 may also be used to provide the signal DB_(ON) with a variable duty cycle. Circuit features giving rise to duty cycle control may be traced from the comparator 321 to the DFF 306.

Comparator 321 compares the triangle signal V_(C), at its non-inverting input, with a reference signal AV_(D) to provide signal D4 to an input of the OR-gate 307. In this way, comparator 321 can assert the signal D4 to logic high when the triangle signal V_(C) exceeds the reference signal AV_(D) and to logic low when the triangle signal V_(C) reduces below the reference signal AV_(D). The OR-gate 307, in turn, can assert the signal R to logic high when the signal D4 is asserted high. Thus, the DFF 306 may reset the signal Q1 to a logic low when the triangle signal V_(C) exceeds the reference signal AV_(D).

To further illustrate, FIG. 4 shows waveform 408 (i.e., signal DB_(ON)) making a transition from low to high at times t1 and t5. However, at times t3 and t7 where the triangle signal V_(C) exceeds the reference signal AV_(D), the signal DB_(ON) transitions from high to low while the oscillator signal DB_(OSC) remains high. In this way the duty cycle (i.e., on time) of DB_(ON) may be varied based on the value of the reference signal AV_(D).

According to the teachings herein, the duty cycle state machine 150 may control and/or change the duty cycle of DB_(ON) based on a system state (e.g., an operating condition). As shown in FIG. 3, the duty cycle state machine 150 may control duty cycle by varying the value of reference signal AV_(D). The duty cycle state machine 150 can provide a control vector (e.g., a plurality of logic signals) DTY to a control input CNT of multiplexor 311. The multiplexor 311, in turn, may select reference signal AV_(D) to be one of a plurality of discrete reference signals AV_(D1)-AV_(D4). For instance, in one embodiment the discrete reference signals AV_(D1)-AV_(D4) may each have unique, discrete value (e.g., voltage) giving rise to a unique, discrete duty cycle. Alternatively, and additionally, in some embodiments there may be greater or fewer than four discrete reference signals AV_(D1)-AV_(D4).

As one of ordinary skill in the art can appreciate, duty cycle state machine 150 can be realized using a variety of circuit approaches including digital and/or analog circuit blocks culling one or more system variables. For instance, as shown in the embodiment of FIG. 3, the duty cycle state machine 150 may use a signal DB_(CLK) and a signal DB_(EN) to determine a system state, and then based on the system state, may provide the control vector DTY. In some embodiments, the duty cycle state machine 150 can use counters to count cycles of signals like signal DB_(CLK).

As shown in FIG. 3 and in FIG. 4, the signal DB_(CLK) may be derived from the clock delay circuit 305 to have a waveform 406 with a rising edge shift delay relative to the oscillator signal DB_(OSC). In providing signal DB_(CLK) the clock delay circuit 305 uses an AND-gate 342 and a delay element 341 to perform a logical AND of the oscillator signal DB_(OSC) with its delayed signal D2. FIG. 4 further illustrates the rising edge shift delay between the oscillator signal DB_(OSC) and signal DB_(CLK). As illustrated, the signal DB_(CLK) has a rising edge shifted by a delay time dt to times t2 and t6 relative to the rising edge of the oscillator signal DB_(OSC). In some embodiments the delay time dt may have a value of 200 ns.

Also, as shown in FIG. 3, the signal DB_(EN) can be provided from RS-latch 310 to indicate when the signal DB_(ON) is enabled and/or inhibited. In making this indication the RS-latch 310 receives the inverted oscillator signal DB_(OSCB) at an input of NOR-gate 329 and receives the signal DB_(ON) at an input of NOR-gate 328. When the signal DB_(ON) is enabled to make clock transitions on a cycle by cycle basis, then the RS-latch 310 may assert the signal DB_(EN) high. Additionally, when the signal DB_(ON) is inhibited (i.e., asserted low), then the RS-latch 310 may assert and maintain the signal DB_(EN) low following a high transition of the inverted oscillator signal DB_(OSCB).

As one of ordinary skill in the art may also appreciate, a gate control module 204 can include greater or fewer components and use fewer or additional signals to generate the signal DB_(ON). For instance, as shown in FIG. 3, DB_(ON) may also be provided and/or inhibited based on an overcurrent condition relating to the sense signal V_(IS). Circuit features relating to overcurrent protection may be traced back to comparator circuit 304. The comparator circuit 304 includes comparator 356 and AND-gate 308. The comparator 356 may assert an overcurrent signal DB_(IS) based on a comparison of the sense signal V_(IS) to a reference V_(R1). Additionally, AND-gate 308 can provide signal D3 to an input of the OR-gate 307. In this way, the comparator circuit 304 can assert the signal D3 to logic high when the delayed clock signal D2 and the overcurrent signal DB_(IS) are coincidentally in a logic high state. The OR-gate 307, in turn, can assert the signal R to logic high when the signal D3 is asserted high. Thus, the DFF 306 may reset the signal Q1 during an overcurrent condition.

FIG. 5 illustrates a state diagram 500 of a duty cycle state machine 150 according to an embodiment of the present invention. The state diagram 500 includes a low state 502, a lower medium state 504, an upper medium state 506, and a high state 508. According to the state diagram 500, a transition from the low state 502 to the lower medium state 504 can occur under a transition condition 512 a, and a transition from the lower medium state 504 to the low state 502 can occur under a transition condition 514 a. Also, a transition from the upper medium state 506 to the high state 508 can occur under a transition condition 512 b, and a transition from the high state 508 to the upper medium state 506 can occur under a transition condition 514 b. Additionally, a transition from the lower medium state 504 to the high state 508 can occur under a transition condition 512 c, and a transition from the upper medium state 506 to the low state 502 can occur under a transition condition 514 c.

Operation of a duty cycle state machine 150, as described herein, may be based, at least in part, upon state diagram 500 for providing the signal DB_(ON) from a gate control module 204 as described above. Additionally, the gate control module 204 may be part of a larger system (e.g., the power conversion system 100 b) having a controller 125 that provides a gate drive signal V_(GB) to a switch (e.g., switch 154) in a switch mode power converter (e.g., switch mode power converter 108). The low state 502, medium low state 504, upper medium state 506, and high state 508 can be power related states indicating how much power is being delivered to a load 110. For instance, the low state 502 can refer to a duty cycle state whereby a duty cycle of the signal DB_(ON) comprises a minimum value causing the switch 154 to transfer least power during a switching cycle. The high state 508 can refer to a duty cycle state whereby a duty cycle of the signal DB_(ON) comprises a maximum value causing the switch 154 to transfer maximum power during a switching cycle. The lower medium state 504 and upper medium state 506 may similarly represent intermediate power states whereby a duty cycle of the signal DB_(ON) comprises an intermediate value causing the switch 165 to transfer power between the least and maximum.

In one embodiment, operation in the high state 508 may correspond to providing the signal DB_(ON) with a maximum duty cycle substantially equal to sixty five percent (i.e., a duty cycle of 0.65). Operation in the low state 502 may correspond to providing the signal DB_(ON) with a minimum duty cycle substantially equal to forty percent of the maximum duty cycle (i.e., a duty cycle of 0.26). Additionally, operation in the lower medium state 504 may correspond to providing the signal DB_(ON) with a duty cycle substantially equal to fifty percent of maximum duty cycle (i.e., a duty cycle of 0.325). Operation in the upper medium state 506 may correspond to providing the signal DB_(ON) with a duty cycle substantially equal to seventy percent of the maximum duty cycle (i.e., a duty cycle of 0.455). With reference to FIG. 3, the duty cycle state machine 150 may control the duty cycle by providing a state dependent control vector DTY to the multiplexer 311. The multiplexer 311, in turn, provides a state dependent reference signal AV_(D) based on control vector DTY.

According to the state diagram 500, the system of the gate control module 204 can enter the low state 502 during a power-up condition 501, which may refer to a condition of applying power to the switch mode power converter 108. Also, the transition conditions 512 a-c and 514 a-c can also be system power related conditions.

With reference to the discussion of FIG. 3, the transition conditions 512 a-c and 514 a-c can be based on a relationship of signals DB_(CLK) and DB_(EN) provided to the duty cycle state machine 150. For instance, the transition conditions 512 a-c can refer to an “enabled consecutive cycle count” condition whereby the signal DB_(CLK) undergoes twelve cycles while the signal DB_(EN) is a logic high. This can also correspond to a condition of delivering power to the load 110 while the signal DB_(ON) is enabled high and while the switch 154 undergoes twelve consecutive cycles. Also, the transition condition 514 a-c can refer to an “inhibited consecutive cycle count” condition whereby the signal DB_(CLK) undergoes twelve cycles while the signal DB_(EN) indicates that the signal DB_(ON) is enabled low. This can also correspond to a condition of inhibiting power transfer to the load 110 while the signal DB_(ON) is inhibited for twelve consecutive cycles.

As described above, one way in which the signal DB_(ON) can be enabled low (i.e., inhibited) and/or enabled high may be traced to a regulation condition of the feedback signal V_(FB). For instance, as shown in FIG. 3, when the feedback signal V_(FB) exceeds the reference V_(R2), then the signal D1 transitions low and the signal DB_(ON) becomes inhibited. By using the duty cycle state machine 150 according to the state diagram 500, the duty cycle may be controlled so that fewer cycles of a constant frequency signal (e.g., signal DB_(ON)) are skipped. In this way, audible noise may be reduced and/or eliminated.

Although FIG. 5 shows a state diagram 500 for controlling duty cycle based on a low state 502, lower medium state 504, upper medium state 506, and high state 508, other state diagrams are possible. For instance, a duty cycle state machine 150 can include fewer or greater discrete duty cycle states and may use transition conditions based on fewer or greater than twelve consecutive cycles. Additionally, the number of consecutive cycles may be chosen based on experiments (e.g., a design of experiments) for reducing audible noise. In some embodiments other state variables may also be introduced into the state machine. For instance, a duty cycle state machine 150 can also receive analog signals, such as a control loop error amplifier output signals, to be used as state variables.

FIG. 6 illustrates a conceptual flow diagram 600 for controlling a switch mode power converter 108 using a duty cycle state machine 150 according to an embodiment of the present invention. The conceptual flow diagram 600 includes a first operation 602, a second operation 604, and a third operation 606. Operation 602 can correspond to delivering a gate control signal (e.g., the signal DB_(ON) from the gate control module 204 and/or the gate drive signal V_(GB) from the controller 125). The gate control signal to the switch 154 may be delivered during an enabled condition relating to regulating output voltage V_(O) relative to an output regulation value (e.g., when the feedback signal V_(FB) is less than a reference V_(R2)). Operation 604 can correspond to inhibiting the gate control signal when the output voltage V_(O) exceeds the regulation value (e.g., when the feedback signal V_(FB) exceeds the reference V_(R2)). Operation 606 can correspond to controlling the duty cycle of the gate control signal (e.g., the signal DB_(ON)) using the duty cycle state machine 150.

The above description of illustrated examples of the present disclosure, including what is described in the Abstract, are not intended to be exhaustive or to be limiting to the precise forms disclosed. While specific embodiments of, and examples understanding for controlling a switch mode power converter using a duty cycle state machine are described herein for illustrative purposes, various equivalent modifications are possible without departing from the broader spirit and scope of the present disclosure. Indeed, it is appreciated that the specific example voltages, currents, frequencies, power range values, times, etc., are provided for explanation purposes and that other values may also be employed in other embodiments and examples in accordance with the teachings herein. 

What is claimed is:
 1. A power conversion system, comprising: a switch mode power converter including a switch coupled to regulate an output voltage to a regulation value in response to a feedback signal; a controller coupled to deliver a gate drive signal to the switch during an enabled control condition, wherein the controller is further coupled to inhibit the gate drive signal to the switch during an inhibited control condition; and a duty cycle state machine included in the controller and coupled to adjust a duty cycle of the gate drive signal according to a plurality of discrete duty cycle states, the plurality of discrete duty cycle states comprising a first state and a second state, wherein during the first state the duty cycle is adjusted to a first value, wherein during the second state the duty cycle is adjusted to a second value, and wherein the duty cycle state machine is coupled to transition from the first state to the second state after the gate drive signal completes an enabled consecutive cycle count during the enabled control condition.
 2. The power conversion system of claim 1, wherein the controller is coupled to deliver the gate drive signal to the switch during the enabled control condition in response to the feedback signal indicating the output voltage is less than the regulation value, and wherein the controller is coupled to inhibit the gate drive signal to the switch during the inhibited control condition in response to the feedback signal indicating the output voltage is greater than the regulation value.
 3. The power conversion system of claim 1, further comprising a transformer, the transformer including a secondary winding coupled to an input of the switch mode power converter.
 4. The power conversion system of claim 1, wherein the switch mode power converter comprises a boost converter.
 5. The power conversion system of claim 1, further comprising: a flyback converter coupled to the switch mode power converter; and a secondary control module comprising the controller coupled to the switch, wherein the controller comprises the duty cycle state machine.
 6. The power conversion system of claim 1, wherein the gate drive signal has a constant frequency.
 7. The power conversion system of claim 6, wherein the constant frequency is substantially equal to two hundred kilohertz (kHz).
 8. The power conversion system of claim 1, wherein the second value is greater than the first value.
 9. The power conversion system of claim 1, wherein the enabled consecutive cycle count is equal to twelve.
 10. The power conversion system of claim 8, wherein the duty cycle state machine is coupled to transition from the second state to the first state after the gate drive signal completes an inhibited consecutive cycle count during the inhibited control condition.
 11. The power conversion system of claim 10, wherein the inhibited consecutive cycle count is equal to twelve.
 12. The power conversion system of claim 1, wherein the plurality of discrete duty cycle states comprises: a high state during which the duty cycle is adjusted to substantially equal a maximum duty cycle; an upper medium state during which the duty cycle is controlled to substantially equal to seventy percent of the maximum duty cycle; a lower medium state during which the duty cycle is controlled to substantially equal fifty percent of the maximum duty cycle; and a low state during which the duty cycle is controlled to substantially equal forty percent of the maximum duty cycle.
 13. The power conversion system of claim 12, wherein the maximum duty cycle is sixty five percent.
 14. The power conversion system of claim 12, wherein the duty cycle state machine is coupled to transition to the low state during a power-up condition.
 15. A method of controlling a switch in a switch mode power supply, the method comprising: delivering a gate drive signal to the switch during an enabled control condition; inhibiting the gate drive signal to the switch during an inhibited control condition; adjusting a duty cycle according to a plurality of discrete duty cycle states, the plurality of discrete duty cycle states comprising a first state and a second state; adjusting the duty cycle to a first value in the first state; adjusting the duty cycle to a second value in the second state; and transitioning from the first state to the second state after counting a first number of consecutive cycles during the enabled control condition.
 16. The method of claim 15, wherein the gate drive signal has a constant frequency.
 17. The method of claim 16, wherein the constant frequency is substantially equal to two hundred kilohertz (kHz).
 18. The method of claim 15, further comprising comparing a feedback signal to a reference signal, the feedback signal indicative of an output voltage of the switch mode power supply.
 19. The method of claim 16, wherein delivering the gate drive signal to the switch during the enabled control condition comprises delivering the gate drive signal to the switch when the feedback signal is less than the reference signal.
 20. The method of claim 16, wherein inhibiting the gate drive signal to the switch during the inhibited control condition comprises inhibiting the gate drive signal to the switch when the feedback signal is greater than the reference signal.
 21. The method of claim 15, wherein the first number of consecutive cycles is twelve.
 22. The method of claim 15, wherein adjusting the duty cycle according to the plurality of discrete duty cycle states further comprises transitioning from the second state to the first state after counting a second number of consecutive cycles during the inhibited control condition.
 23. The method of claim 22, wherein the second number of consecutive cycles is twelve.
 24. The method of claim 15, wherein the plurality of discrete duty cycle states comprises a high state, an upper medium state, a lower medium state, and a low state, wherein adjusting the duty cycle according to the plurality of discrete duty cycle states further comprises: adjusting the duty cycle to a maximum duty cycle in the high state; adjusting the duty cycle to substantially equal seventy percent of the maximum duty cycle in the upper medium state; adjusting the duty cycle to substantially equal fifty percent of the maximum duty cycle in the lower medium state; and adjusting the duty cycle to substantially equal forty percent of the maximum duty cycle in the low state.
 25. The method of claim 24, wherein the maximum duty cycle is sixty five percent.
 26. The method of claim 24, wherein adjusting the duty cycle according to the plurality of discrete duty cycle states further comprises transitioning into the low state during a power-up condition.
 27. A switch mode power supply system for controlling an output voltage delivered to a load, the switch mode power supply system comprising: a switch mode power converter comprising a switch coupled to regulate the output voltage; and a controller coupled to provide a constant frequency gate signal to the switch when the output voltage is less than a regulation value and to inhibit the constant frequency gate signal to the switch when the output voltage is greater than the regulation value, wherein the controller comprises a duty cycle state machine coupled to control a duty cycle during a cycle of the constant frequency gate signal and a gate control module, the gate control module comprising a multiplexer coupled to provide a state dependent reference signal selected from a plurality of discrete reference signals.
 28. The switch mode power supply system of claim 27, wherein the gate control module comprises a variable duty cycle square wave generator coupled to provide a square wave in relation to a triangle wave, wherein the square wave has a duty cycle based, at least in part, upon the state dependent reference signal, and wherein the constant frequency gate signal is proportional to the square wave.
 29. The switch mode power supply system of claim 28, wherein the controller comprises a buffer coupled to the variable duty cycle square wave generator to buffer the square wave so as to provide the constant frequency gate signal.
 30. The switch mode power supply system of claim 28, wherein the gate control module comprises a triangle wave generator coupled to provide the triangle wave to the variable duty cycle square wave generator.
 31. The switch mode power supply system of claim 30, wherein the gate control module comprises the duty cycle state machine coupled to provide a control vector to the multiplexer based on a system state. 